1. Field of the Invention
The present invention relates to an automatic exposure control apparatus with improved exposure value calculation for a moving object. More particularly, the invention relates to an apparatus which calculates an exposure value so as to account for predicted movement of a moving object (and thus predicted change of size of the object within a phototaking image plane) between a time of photometering and a time of exposure, thereby to obtain an optimum exposure of the object.
2. Related Background Art
As a conventional automatic exposure control apparatus, there is known, for example, an apparatus which calculates an exposure value as described below.
As shown in FIG. 9, a field is divided into six areas P1 to P6. Photometering is performed for each area. Based on the values obtained by this photometering, exposure information used for an exposure value calculation is determined by the following equations EQU SP=B6 EQU CW=(B5+B6)/2 EQU BM=(B1+B2+B3+B4+B5)/5 EQU BLM=(BM+Bmin)/2 EQU BHM=(BM+Bmax)/2
where B1 to B6 are each a brightness; Bmin is a minimum brightness among brightnesses B1 to B6; Bmax is a maximum brightness; SP is exposure information determined by spot photometering only the central area P6; and CW, BM, BLM, and BHM are exposure information calculated by multi-pattern photometering using selected ones of the areas P1 to P6.
As to the multi-pattern photometering, CW is exposure information in which a weight is given to central areas P5 and P6 (hereinafter referred to as center-weighted photometering); BM is exposure information based on the average value of each of the areas P1 to P6; BLM is exposure information in which a weight is given to a low brightness; and BHM is exposure information in which a weight is given to a high brightness.
One of these kinds of exposure information is selected on the basis of the maximum brightness Bmax, and a brightness difference (Bmax-Bmin) between the maximum brightness Bmax and the minimum brightness Bmin. An exposure value is calculated on the basis of the selected exposure information. A shutter and a diaphragm are driven on the basis of this exposure value and a photograph is taken.
In addition, as disclosed in Japanese Patent Laid-Open No. 1-154133, a known apparatus determines an optimum exposure on the basis of information about a distance to an object obtained by a range finder and the brightness of each area obtained by the multi-pattern photometering described above, by taking into account, for example, the object distance and the brightness distribution condition of the field. For example, when there is a high brightness in P1 or P2 among the above-mentioned areas P1 to P6, this brightness is judged to be caused by the sun. Photometering outputs of P3 to P6 excluding the outputs of P1 and P2 are used to determine the exposure value. When areas P5 and P6, or area P6, are darker than the other areas, it is judged that a main object, e.g., a person, is against the light, and photometering outputs of the areas P5 and P6, or area P6, are used to determine the exposure value. Further, optimum exposure information is selected based on the distance to the object even if the brightness distribution of the field has the same pattern in the case where the main object is a person at a object distance of 3 m or a scene at an object distance .infin..
In a conventional automatic exposure control apparatus, a problem arises in that even if an exposure value is calculated on the basis of the optimum exposure information at a photometering time for a moving object, i.e., an object approaching or moving away from the camera, the exposure value is not an optimum value at the exposure time, because the size of the object within the phototaking image plane at the exposure time differs from that at a photometering time due to the time lag between the photometering time and the exposure time.
FIG. 10 shows how a phototaking lens is driven by means of an automatic focus control apparatus (hereinafter referred to as an AF apparatus) to follow an object moving closer to a camera. The image forming plane position where an image is formed by the phototaking optical system changes as indicated by the straight line X, and the phototaking lens position changes as indicated by the stepped line Y.
In FIG. 10, reference character Y1 denotes a time period in which range finding is performed by the focus detection optical system and a range finding element. During the same time period, photometering is performed by a light-receiving element. Reference character Y2 denotes a time period during which a defocus amount is calculated on the basis of the output from the range finding element; and reference character Y3 denotes a time period during which a phototaking lens is driven on the basis of the calculated defocus amount. The phototaking lens is driven by a drive amount D1 during this time period Y3 and reaches a position X1. However, at this time, the image plane position of the object has already moved to X2. Therefore, range finding is performed again during a time period Y4. In a time period Y5, the AF apparatus adds a drive amount D2' of which the object is predicted to move by the next range finding time period Y7 to a lens drive amount D2 proportional to the defocus amount obtained by the range finding performed again and drives the phototaking lens by the drive amount (D2+D2') in the time period Y6. As a result, it is put in focus at X3 immediately before the next time period Y7. If the shutter release is operated in a period from Y1 to Y6, an exposure is performed after the phototaking lens is driven (Y6) and an in-focus photograph can be taken.
FIG. 6 is a view showing an object image, specifically, a person's image, within the phototaking image plane as the person moves closer to the camera in the manner shown in FIG. 10. The solid line in the figure indicates the person's image at X2 in FIG. 10. The person's image increases in size within the phototaking image plane with the passage of time and becomes the size shown by the dashed line at a focus position X3.
Suppose, for example, that photometering is performed during a period Y4 and an exposure value is calculated on the basis of the photometering value of each photometering area. At this time, the person's image does not occupy a large area within the phototaking image plane, as shown by the solid line of FIG. 6. Hence, multi-pattern photometering in which an exposure value is calculated on the basis of the average value of brightnesses of each area is performed. The actual exposure, however, is performed on the basis of this exposure value at the later in-focus time corresponding to the position X3; and, as described above, the size of the person's image within the phototaking image plane at the exposure time becomes the size indicated by the dashed line in FIG. 6. Therefore, center-weighted photometering (P5+P6) or spot photometering (P6) would be preferable to using the average value of brightnesses.